Phosphorescence in Mn⁴⁺-Doped R⁺/R²⁺ Germanates (R⁺ = Na⁺ or K⁺, R²⁺ = Sr²⁺)

Single crystals of three new compounds, Na_0.36Sr_0.82Ge₄O₉ (1, proposed composition), Na₂SrGe₆O₁₄ (2), and K₂SrGe₈O₁₈ (3), were obtained and characterized using single-crystal X-ray diffraction. Their structures contain three-dimensional (3D) anionic frameworks built from GeO₄ and GeO₆ polyhedra. The presence of octahedral Ge⁴⁺ sites makes the new phases suitable for Mn⁴⁺ substitution to obtain red-emitting phosphors with a potential application for light conversion. Photoluminescence properties of Mn⁴⁺-substituted Na₂SrGe₆O₁₄ (2) and K₂SrGe₈O₁₈ (3) samples were studied over a range of temperatures, and red light photoluminescence associated with the electronic transitions of tetravalent manganese was observed. The Na₂SrGe₆O₁₄ (2) phase was also substituted with Pr³⁺ on the mixed Na-Sr site similar to the previously studied Na₂CaGe₆O₁₄:Pr³⁺. The red emission peak of the Pr³⁺ activator occurs at a shorter wavelength (610 nm) compared to that of Mn⁴⁺ (662-663 nm). Additionally, second harmonic generation (SHG) data were collected for the noncentrosymmetric Na₂SrGe₆O₁₄ (2) phase, indicating weak SHG activity. Diffuse reflectance spectroscopy and density of states calculations were performed to estimate the band gap values for pristine Na₂SrGe₆O₁₄ (2) and K₂SrGe₈O₁₈ (3) phases.